1. Field of the Invention
The present invention relates generally to electrical circuits, and more particularly, to circuits and techniques for broadband amplifier linearization.
2. Description of the Related Art
Efficient transmission of broadband information such as analog CATV and digital QAM (quadrature amplitude modulation) signals requires linear amplification of content throughout networks. Linear amplifiers are those which add low amounts of distortion as they increase the amplitude of a signal. Amplifiers which add excessive distortion cause degradation of picture quality and reduced BER (bit error rate). Maintaining good distortion performance throughout the system requires careful attention to the design of amplifiers along the transmission chain.
Amplifiers in CATV systems generate two primary types of distortion; 2nd order and 3rd order. Each type of distortion leads to unique system impairments that limit the effective amount of content or information that can be transmitted through the system. For example, 3rd order distortion in an amplifier causes QAM signals to develop spectral content which can impair the BER on the adjacent QAM channel. This spectral re-growth is a primary concern in amplifiers for digital RF networks.
Second order distortion impacts a CATV system is a number of ways, such as analog video picture quality. Increasingly, in new CATV architectures, however, the presence of 2nd order distortion is a concern when both analog channels and QAM content are transmitted at the same time through an amplifier. Usually the QAM content is located in the upper frequency band of the system and the analog channels are left in the lower band. Second order distortion mechanisms occurring in the upper band will generate difference products that fall back into the lower frequency analog band. Since the original QAM content in the upper frequency is un-correlated to the analog content and therefore noise-like, the 2nd order distortion will manifest as an increase in the noise floor of the analog signal, thereby degrading picture quality.
The issue is heightened by the fact that newer CATV architectures strive to reduce the number of system amplifiers by outputting higher levels of RF signal with higher levels of tilt. Tilt is the difference between the signal level at the highest channel compared to that at the lowest channel. The higher output levels and increased tilt means the RF power located in the QAM band is higher in new systems and the level in the analog band is lower. This further exasperates the 2nd order distortion issue and leads to significant design challenges in the design of CATV amplifiers. Increasing care must be taken to insure that 2nd order distortion products do not limit CATV systems. In particular, high QAM levels must not be allowed to cause lower analog channels to have carrier-to-noise (CNR) issues. Considering the newer QAM content and higher RF outputs, it's apparent many of the prior art linearization techniques did not did not address this issue with sufficient care.
There are a few common approaches to controlling and reducing the distortion in amplifiers. The simplest technique is to increase the size of the transistors used in the amplifiers themselves. While this readily drops the distortion, it also leads to higher power consumption since the larger transistors require more power to operate. Another technique is to use newer transistor technologies which at the device level are inherently more linear. Recently much work has been done to improve the internal distortion of transistors. Often these newer transistor technologies pose significant reliability risks as they reach technical maturity.
A prior art approach to pre-distortion are shown in FIG. 1. Here an amplifier with distortion needing to be reduced is driven from a pre-distortion circuit. For linearization to occur, the distortion created by the pre-distortion circuit needs to be the same level and opposite phase as the distortion occurring in the amplifier. In other words, the distortion from the linearizer, if it were amplified by a distortion-free amplifier, would need to have the same magnitude but opposite phase as the distortion occurring from the real amplifier itself when operated at the same output levels. Hence, operation of the circuit in FIG. 1 can be thought of as a destructive interference circuit, where the goal is to achieve good alignment of both the magnitude and phases of the two distortion signals so there is as much cancellation as possible.
One important goal of amplifier design is to achieve good bandwidth with devices which themselves often have significant terminal capacitances and charge storage effects, hereafter called capacitances, which slow down the movement of charge in the circuit and restrict bandwidth. It is also critical to point out that these device capacitances are themselves non-linear functions of the terminal voltages and currents. For low frequencies these effects have minimal influence on amplifier distortion. However, at higher frequencies, these capacitances can dominate amplifier distortion.
The design of the amplifier in FIG. 1 is usually performed with a few familiar topologies, as shown in FIG. 2. Differential versions of these topologies are also commonly used. The common-emitter topology is known to have good efficiency and noise performance, and is easy to design. However, it tends to have poor performance because device capacitances can dominate the frequency response and high frequency distortion.
The Cascode topology helps alleviate the input to output capacitance, commonly known as the collector-base or “Miller” capacitance by placing a 2nd transistor above the main transistor. The Cascode or top device's emitter provides low impedance to the bottom device, which prevents build up of voltage and minimizes the necessary energy to charge and discharge the collector-base capacitance. The Cascode is by far the dominant topology used in infrastructure CATV amplifiers today. Even still, the Cascode amplifiers used with older silicon technology still have significant degradation in their distortion performance as frequency increases.
The Darlington topology shown in FIG. 2 also has increased bandwidth. The 1st device acts as a buffer stage that has unity gain up to the input base of the 2nd device. The buffer is a drive device which is able to move charge in and out of the 2nd devices' capacitances. Consequently, the Darlington topology can have very good bandwidth. Note the Darlington shown in FIG. 2 still suffers from the Miller effect and likewise has significant degradation in distortion performance at higher frequencies.
All amplifier topologies in FIG. 2 will have some degradation in distortion performance as frequency is increased. The drop off in distortion performance of the amplifier with increasing frequency indicates that the magnitude of the amplifier's distortion is increasing. It also suggests that the phase of the distortion is also not constant with frequency. This change in magnitude and phase of the distortion signal as frequency is increased makes the design of the linearizer much more difficult.
Normally during the design of the amplifiers shown in FIG. 2 the type of frequency response of the distortion products is of little concern. For broadband amplifiers, the design goal is usually to have an amplifier whose composite distortions be minimized. In the field of CATV amplifiers, relevant distortions are composite second order (CSO) and composite triple beat (CTB). Normally the design goal for a CATV amplifier would be for the CSO and CTB to be lower than some specified value. There is usually little concern given with regard to which frequencies the CSO and CTB are lowest.
A common shunt-type linearizer circuit is shown in FIG. 3a. Here, two weakly forward biased diodes, D1 and D2, are placed in anti-series and across the input signal line. The capacitors C1, C2, and C3 are all large bypass capacitors. R1, R2, and IBIAS are used to set the bias condition in the diodes. In normal operation D1 and D2 are weakly biased at a very small current. FIG. 4 shows the IV characteristics of the forward biased diode. For common diodes the relationship between terminal voltage and current is given by an exponential relationship. This exponential may be differentiated to give the effective video resistance, or small signal resistance, of the diode at the specific IBIAS. Note that if the amount of forward bias current is increased from IBIAS, the video resistance dynamically decreases, and that if the current is likewise decreased the video resistance is dynamically increased. Thus, when an incident signal is applied to the diode having a bias current of IBIAS the diode acts as a resistor whose resistance varies with the incident signal. Considering the circuit of FIG. 3A again, an RF signal on the input will both increase and decrease the node voltage at point A in the circuit. When the voltage at node A increases, the currents in D1 and D2 both change. The forward bias current in D1 increases, which causes its video resistance to decrease. However, the forward bias current in D2 decreases, which causes its video resistance to increase. Because of the exponential behavior of the diode, the increase in D2's video resistance is greater than the decrease in D1's video resistance, which means the overall video resistance of D1 and D2 in series increases as the RF voltage at point A increases. This means the loss in the linearizer circuit decreases as the RF voltage increases. This is commonly known as “gain” expansion.
There are many pre-distortion circuits but their operation is similar to FIG. 3. A diode's exponential characteristics are used to make a circuit which has less loss when the input signal level is high. This characteristic is almost always what is needed because most amplifiers have the opposite gain compressive behavior as the RF input is increased.
A series-type linearizer is shown in FIG. 3B and is used in numerous patents in the CATV industry, such as U.S. Pat. No. 6,107,877 (Miguelez, et al) and U.S. Pat. No. 6,580,319 (Cummings, et al). Miguelez and Cummings both apply series-type pre-distortion to the familiar CATV hybrid amplifier, which is thought of as the workhorse amplifier component in CATV distribution networks. Operation is similar to the earlier described shunt-type, where an increase of the incident RF signal leads to a decrease in the amount of loss. In this case, an increase in RF signal leads to a drop in the dynamic diode resistance; in the series path a drop in resistance leads to an increase of gain.
The circuits in FIGS. 3A and 3B are commonly used to pre-distort amplifiers. Alignment of its distortion to that of the amplifier is often done by adjusting the value of IBIAS. Higher values of IBIAS will lead to less distortion from the linearizer circuit. Note the circuits in FIGS. 3A and 3B allow for only the adjustment of the magnitude of the distortion term, with no ability to control the phase. As noted, most amplifiers' distortion products have varying phase response over frequency, so the circuits in FIGS. 3A and 3B are incomplete.
Often designers place phase compensation circuits to align the phases for best cancellation. Miguelez and Cummings disclose techniques for adjusting the phase of the pre-distortion signals for cancellation with the amplifier's distortion. Small capacitance or inductance may be used to perform the alignment. However, the circuits disclosed in Miguelez and Cummings are not easily implemented in an integrated circuit because they require capacitors of large value to achieve the low frequency response needed. The added capacitors will tend to reduce the possible benefits of integration due to their added parasitic length to the circuit. In short, the series-type linearizers of Miguelez and Cummings do not lend themselves to integration.
A similar statement may be made about U.S. Pat. No. 5,798,854 and U.S. Pat. No. 6,288,814, (both Blauvelt), where a real and imaginary alignment of distortion terms with respect to electronic or optical elements is disclosed. The techniques disclosed in Blauvelt require either large blocking capacitors or biasing inductors to route biasing signals around or to diodes or FETs.
Techniques commonly used to pre-distort optical transmitters often make use of broadband splitters or couplers to divide signals in specific ratios. They also rely on delay lines to help align the phases between pre-distorters and amplifiers or optical transmitter. A few prior art references that illustrate this are U.S. Pat. No. 5,589,797 (Gans, et al) and U.S. Pat. No. 5,436,749 (Pidgeon, et al), which both utilize delay lines and are hence not suitable for integrated circuit implementation.
U.S. Pat. No. 5,282,072 (Nazarathy, et al) discloses a shunt-type linearizer for optical transmitters which may be extended to work with amplifiers. However, the circuitry disclosed in Nazarathy is not well designed with respect to their suitability in an integrated circuit process with uncertain matching characteristics. Any imbalance in the diode characteristics will lead to imbalance of currents in the diode branches and potentially large 2nd order distortion. The potential for large 2nd order distortion makes Nazarathy of questionable value for integrated designs. Furthermore, Nazarathy does not disclose the critical step of aligning the phase responses between the pre-distortion generator and amplifier, which as earlier noted can require large delay lines and greatly complicate the task of integration. Modifications on Nazarathy require the aforementioned large blocking capacitors or bias inductors, which are likewise not given to integration.
One problem with the pre-distortion circuit in FIG. 3A is the premature clipping of the linearizer before the amplifier compresses completely. In other words, distortion characteristics from common linearizers, such as that in FIG. 3A, vary considerably as the input RF level is increased, and high amounts of RF drive can cause the linearizer to clip prematurely before the amplifier does. This leads to very undesirable degradation of combined compression performance. Hence, it becomes very important to design pre-distortion circuits that have the necessary distortion characteristics to insure good cancellation with the amplifier, but which do not prematurely clip as the input drive is increased. Very often in prior art linearizers the pre-distortion circuit adversely affects the high power performance of the combined response.
Finally, U.S. Pat. No. 5,172,068 (Childs) discloses how multiple series diodes in an anti-series configuration may be used as a pre-distortion circuit for 3rd order products. Childs discloses that multiple series diodes makes it possible that a larger RF signal may be sent through the circuit. Childs does not show a method for adjusting the phase response of the distortion terms, only how their variation over frequency may be minimized. Instead, Childs discloses that a higher RF level can be useful in maintaining good high frequency distortion cancellation characteristics. Childs otherwise does not show how the phase alignment between the pre-distortion circuit and the element being linearized may be adjusted other than by adjusting the number of diodes and the input RF level.